1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method and more particularly, to an image forming apparatus and an image forming method in which an image is formed by ejecting a light-curable ink from an inkjet head.
2. Description of the Related Art
As an image forming apparatus, an inkjet printer (inkjet recording apparatus) is known, which comprises an inkjet head (liquid droplet ejection head) having an arrangement of a plurality of nozzles (liquid droplet ejection ports) and which forms images on a recording medium by ejecting droplets of ink from the nozzles toward the recording medium while causing the inkjet head and the recording medium to move relatively to each other.
According to an inkjet recording printer, one image is formed on a recording medium by combining ink dots created by ink ejected from the nozzles. In recent years, it has become desirable to form images of high quality on a par with photographic prints, by inkjet printers. It has been thought that high image quality can be achieved, for example, by reducing the size of the ink droplets ejected from the nozzles by reducing the diameter of the nozzles and increasing the number of pixels per unit surface area by arranging the nozzles at high density.
On the other hand, the apparatus becomes larger in size if the nozzles are arranged at high density and a large number of nozzles are provided, and therefore it is also necessary to reduce the size of the apparatus. Furthermore, since inkjet recording apparatuses which require special paper place limitations on the recording medium that can be used, there have also been demands to achieve high-quality image recording, regardless of the type of recording medium.
For example, Japanese Patent Application Publication No. 2004-42548 discloses technology in which, in a case where a recording medium is conveyed by a medium conveyance belt which is wound around a drive roller and an idle roller, or the recording medium is conveyed by rotating the recording medium in a wound state around the surface of a rotating drum, and ultraviolet-curable ink is ejected at prescribed staggered timings from different nozzles, thereby recording dots onto the recording medium. After that, ultraviolet light is radiated onto the dots of ultraviolet-curable ink ejected from the nozzles onto the recording medium, in accordance with the ejection timings, thereby raising the viscosity of the ink and pre-curing the ink to a level whereby adjacent dots do not mix with each other. Then ultraviolet light is radiated again so as to perform main curing, thereby preventing bleeding (landing interference).
Furthermore, for example, Japanese Patent Application Publication No. 2003-251910 discloses an inkjet recording method in which, when printing is performed onto a substrate by ejecting ink that is curable by at least one of either irradiation of radiation or heat energy, from a recording head having at least one nozzle capable of selective control of the ejection of ink droplets, an ink containing two or more types of photoinitiator of different light absorption wavelengths is used, and furthermore, when the ink is cured by the irradiation of radiation, the irradiation is divided into two stages using different wavelengths or intensities, in order to improve the adhesion and the conformity of the ink film with respect to the substrate, and in particular, two or more types of initiator having different light absorption wavelengths are used conjointly.
Moreover, Japanese Patent Application Publication No. 2001-129982 discloses an inkjet printer in which, when paper is conveyed via a rotating drum and ink is ejected from an inkjet head, if there are ink dots to be ejected which are mutually adjacent, then within a plurality of ink ejections, a previously established ejection waiting time (for example, the time required for one rotation of the drum) is inserted before ink ejection for a dot which is adjacent in the main scanning direction or the sub-scanning direction is performed.
Furthermore, Japanese Patent Application Publication No. 2001-1512 discloses an inkjet printer which comprises: an image forming unit that ejects ink onto sheet-shaped paper (printed object); and a pressurizing unit which pressurizes the printed object on which an image has been formed by means of the image forming unit. Such a printer is constituted in such a manner that a luster is applied to the image by pressurizing and flattening the surface undulations of the ink of the image formed on the printed object by the image forming unit.
However, in the related art technology described above, there are still problems when high-quality recording is performed at high speed, onto any desired recording medium.
For example, in the technology described in Japanese Patent Application Publication No. 2004-42548, droplets of an ultraviolet-curable ink are ejected onto a rotating drum and pre-cured (semi-cured), separately for each color, and main curing is carried out after droplet ejection and pre-curing has been completed for all of the colors; however, since the droplets of the ultraviolet-curable ink are ejected directly onto a recording medium wound around a rotating drum, then the ink which has not yet been pre-cured permeates into the recording medium immediately after droplet ejection, and hence bleeding may occur, depending on the type of recording medium used.
Furthermore, according to the inkjet printer in Japanese Patent Application Publication No. 2001-1512, a luster is applied to the image by pressurizing and flattening the surface undulations of the ink of the image, in respect of the dots formed by ultraviolet-curable ink; however, when this flattening process is carried out, the ink curing reaction of the droplets of the previously ejected color can be in a fully cured state, and problems may occur, such as peeling or cracking of the ink surface, during the pressurization and flattening process.
Furthermore, one possible method for forming images in order to obtain high-quality recording on any desired medium, regardless of the permeability of the recording medium, is an intermediate transfer method; however, in consideration of the required transfer characteristics of ultraviolet-curable ink onto the recording medium, it is necessary to maintain a semi-cured state until the ink deposited on the intermediate transfer drum has been transferred onto the recording medium.
Furthermore, in order to prevent landing interference and to prevent complication of the control system, it is necessary to eject droplets onto the intermediate transfer drum and semi-cure the droplets, separately for each color; however, the ink curing reaction of the previously ejected droplets progresses and they achieve a fully cured state, and therefore such transfer becomes impossible.
For example, if an intermediate transfer method based on a rotating drum is applied in the system described in Japanese Patent Application Publication No. 2004-42548, then the curing reaction can proceed too far in the semi-curing process, and the ink can reach a fully cured state, thus giving rise to transfer errors. In particular, if droplets of an ultraviolet-curable ink are ejected onto a rotating drum separately for each color, then there is a possibility that the curing reaction of the ultraviolet-curable ink deposited previously onto the rotating drum progresses and leads to main curing.
Moreover, according to the inkjet recording method described in Japanese Patent Application Publication No. 2003-251910, the ink contains two types of initiators of different light absorption wavelengths, and light is radiated in two stages by means of two types of ultraviolet light sources having different wavelengths; however, the object of this method is to improve the adhesion and compliance of the ink film with respect to the substrate, rather than preventing landing interference and guaranteeing transferability. Furthermore, the inkjet printer described in Japanese Patent Application Publication No. 2001-129982 does not aim to achieve both high-quality recording and high-speed recording. Neither Japanese Patent Application Publication No. 2003-251910 nor Japanese Patent Application Publication No. 2001-129982 achieve high-quality image recording at high speed on any recording medium, regardless of the permeability of the recording medium, by means of an intermediate transfer method.